The present disclosure relates to a focus detection unit applicable to an optical apparatus such as a microscope, and to the optical apparatus.
Techniques for accurately detecting a focus have been studied in optical devices such as measuring microscopes. For example, Japanese Patent Application Laid-open No. Hei 10-142515 discloses a focus detection unit using an interposing system. The focus detection unit forms an image of sight double beams and a sight single beam on an image surface of a microscope optical system, projects the image onto an object to be observed, and adjusts the sight single beam to be sandwiched at the center of the sight double beams, thus obtaining a focal point. As shown in FIG. 1 of Japanese Patent Application Laid-open No. Hei 10-142515, the sight double beams and the sight single beam are separated from each other using a prism 18. This can simplify the optical system and form a compact focus detection unit, as compared with the interposing system in related art (Japanese Patent Application Laid-open No. Hei 10-142515, paragraphs [0024] to [0026]).